May First is perhaps the most holiday-rich day in the calendar. It is a cross-quarter day along with November 11 falling between the March and September equinoxes respectively and is associated with northern European pagan and neo-pagan celebrations, notably the May Pole, which is variously and inconclusively considered to be an axis mundi and a phallic image.
In many parts of the world, including China where I live, it is International Labor Day and is a mandatory day off. During the Cold War, the United States and other countries sought to take the focus off the labor movement and have made it Law Day, Loyalty Day, Child Health Day, and at least once in recent history, it was International Day of Prayer.
It is also Lei Day in Hawaii, Maharashtra Day in India, Constitution Day in Argentina, Latvia, and the Marshall Islands (coincidence, as far as I can tell.)
But my personal favorite is International Sunflower Guerrilla Gardening Day. In case you’ve never heard of guerrilla gardening, it is the act, sometimes surreptitiously, of planting flowers or vegetables in abandoned sites, areas that are not being cared for, and/or private (other than their own) property.